This invention relates to blends of ethylene vinyl alcohol copolymer with a minor amount of amorphous polyamide component and their use as a barrier layer in thermoformed multilayer containers and other applications.
Blends of ethylene vinyl alcohol polymers with polyamides in general are known, and have been used in packaging applications as barriers to inhibit the passage of atmospheric oxygen or other gases.
Japanese patent application 53-49050 discloses a blend of EVOH with 5 to 40 weight percent polyamide. The polyamides include a copolymer of hexamethylene diamine with isophthalic and terephthalic acids, in mole ratios of 100/0 to 50/50. The blend is formed into a film, which is said to possess excellent gas barrier properties.
U.S. Pat. No. 3,726,034, Bottenbruch et al., discloses mixtures of 70-99% polyamide and up to 30% of a hydroxyl containing polyolefin. The polyamides "consist of linear unbranched polymer chains containing no additional functional groups." Exemplified are blends of nylon 6 and EVOH.
U.S. Pat. No. 4,079,850, Suzuki et al., discloses a multilayer blow molded container, which contains a layer which may be EVOH, polyamide, or various blends, providing gas barrier properties. The polyamides which are mentioned are nylon 6, nylon 66, and nylon 12.
U.S. Pat. No. 4,427,825, Degrassi et al., discloses a composition of matter useful for making films, of polyamide and 1-65% EVOH. Nylons with melting points greater than 175.degree. C. are preferred, such as nylon 11 or nylon 12.
U.S. Pat. No. 4,500,677, Maruhashi et al., discloses a resin composition comprising a mixture of two EVOH resins and a polyamide resin. The ratio of the EVOH resins to the nylon resin can be between 95:5 and 5:95. Nylon 6, nylon 6,6 and other polyamides having "linear alkylene group[s]" are specifically mentioned.
Ethylene vinyl alcohol copolymer (EVOH) is commonly used in the form of a thin layer together with thicker layers of less expensive structural materials, for example, polypropylene or polyethylene terephthalate, in order to form a structure which is resistant to the passage of atmospheric oxygen or other gasses. In order to make containers or oriented films of such multi-layer structures a solid phase thermoforming process is often used. However, EVOH cannot be formed at the relatively lower temperatures required for the thermoforming of many common structural materials without rupturing the EVOH layer, resulting in a loss of overall barrier performance. The present invention provides a modified EVOH composition which may be used in thermoformed multilayer structures to avoid the above mentioned problems, and without substantially sacrificing the excellent gas barrier properties of EVOH. It may also be used in other applications where stretching is required during the processing of the article, such as in shrink films.